[Mp4-tech] about decoding incomplete h264 video with frame lost

[hong liu]

I have been working on h264 streaming video quality accessment. I use mplayer to receive h.264 videos
streamed from a Darwin streaming server. MPlayer use ffmpeg to decode h.264 videos. So I have some 
questions with regard to the ffmpeg in decoding h264 videos. 
Now I use JM12.2 of the h264 test model to encode 10 frames of a 4:2:0 raw video sequence named test.yuv to an h.264 video named test.264. The Intra interval is 5 frames and no B frame is enabled for
this testing. So the 10 frame types are listed: I0 P1 P2 P3 P4 I1 P5 P6 P7 P8, where I is denoted I frame
and P is P frame. 
Now I manually deleted P3 frame from the H264 encoded video file, test.264 to save it as
test_damage.264. After ffmpeg was used to decode test_damage.264, 9 frames of a raw video sequence
were generated, named as test_damage.yuv. Then I analyzed these two raw video sequences: test.yuv
and test_damage.yuv. I found I0 P1 P2 are the same in both sequences. I1 is the same also. all the left
frames are different. 
My questions  are arised from this result. 
1. Because the P3 frame was deleted manually, so ffmpeg decoder may not decode the  p2 frame
    properly. I am not sure about how the ffmpeg respond to this lost frame. Will it do any error
   concealment on this lost frame. From the analysis result, it may not do anything on it. Under this
   assumption, the p4 frame was decoded, but it was corrupted due to the loss of the P2 frame. Is that true
   for ffmpeg decoder? 
2. The second question confused me a lot for a
 while. Because  I1 P5 P6 P7 P8 are intact, these decoded
    uncompressed video frames should be the same as the original ones. Why is there only I1 are found
    exactly the same as the original one? Is encoding P5 depends on not only I1, but also P4? To my 
    understanding, decoding the frames of I1 P5 P6 P7 P8 would not be affected by any frame lost from
   I0 to P4, because their encoding is not dependent on the former video frames. I need an video expert to
   correct me!
Any help is very appreciated and thank you in advance!
Hong Liu
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